DIY 12AU7 (ECC82) Tube - IRF612 MOSFET Hybrid Headphone Amplifier

I have always been intrigued by tube amplifiers, but most DIY kits are very expensive and use very high voltage. So I decided to build an amplifier that would be inexpensive and had the least amount of parts necessary to drive a pair of 32 ohm Grado headphones.

Having built several YAHA amps based on the fa-schmidt design, and a Szekeres Mosfet follower I wondered how the two would sound together. So I built the schematic into TINA-TI, a free spice based program to test circuits before the build, and the results were remarkable. Nearly 20dB of gain across 20Hz-100kHz from a 13VDC power supply.

Figure 1: 12AU7 / IRF612 Mosfet Frequency Response Simulation

As you see in the schematic and parts list, there are less than 30 discrete components and most DIY'ers will have them as spares from other builds. I chose the 12AU7 / ECC82 vacuum tube because it can be driven with low voltage and the filament voltage is 12.6 volts, so there is no need to regulate the voltage any further. I used 1/4W resistors in the first stage and 2W in the second. The 2W resistors may be overkill but I did not want to change them later. The 20ohm resistor must be a minimum of 5W and do not use wire wound, as the inductive characteristics will distort the response curve.

Heatsink must dissipate around 3W so 2.5-3 square inches, or less if you use a fan. Use heatsink paste and mica insulators as well as insulated washer. The tab of the Mosfet is at (12-13vdc)

Chassis of your choice, plan for ventilation.

13VDC switching power supply, can be between 12 and 13 volts to heat the filaments. Power jack is a generic DC jack from Radio Shack.

The MOSFET (T1) can be replaced by an IRF510, IRF610 or IRF611.

Construction - Tube / MOSFET Hybrid Headphone Amp

For the enclosure I used a Lansing MicroPak "C"; which I found on eBay for approx $8. This item was an overstock of some surround decoders and came with RCA connectors and pre-punched faceplates. The tube socket should be a 9 pin, this one was for older pixie tubes so there are more holes, but the 9-pin match perfectly.

Photograph 1: Lansing MicroPak Enclosure and PCB

I used RCA connectors for input and a 1/4" jack for an output, this was to accommodate my Grado SR125's and the holes in the plates were punched for a 1/4" jack.

The PCB had pre-existing solder runs and a ground bus so this dictated the location of the components If you build this on proto board make sure to leave room for the heatsink and the 20 ohm resistors to dissipate heat. Make sure to insulate the Mosfet from the heatsink with mica, and also use heatsink compound for better heat transfer. The tab of the Mosfet is at 12-13 volts and will destroy the power supply and component if it shorts to ground.

Photograph 2: Construction of Hybrid Headphone Amplifier

After all of the components were soldered I smoke tested the unit and made any necessary corrections.

Photograph 3: Tube / Mosfet Hybrid Amplifier Smoke Test

The tube heater draws 150mA at 12.6 and the whole amp draws 580 mA at startup and stabilizes at 550mA after the tube warms up; which takes a few seconds. I used a Canon AD-360U switching power supply from their small bubble jet printer. Since the drain of the Mosfet is connected directly to the positive rail any noise will be amplified. This power supply had virtually no noise.

Photograph 4: Hybrid Headphone Amplifier - Front View

Photograph 5: Hybrid Headphone Amplifier - Rear View

I used a 12V muffin fan dropped to 9V to cool the amp, two holes in the top allow air to rush in and the existing holes in the front and rear plate allow the air to flow well.

Photograph 6: Tube / Mosfet Hybrid Headphone Amplifier

Measurements - Tube / MOSFET Hybrid Headphone Amp

Here are a few scope shots of the 10 Hz sine wave response, as well as the square wave response at 100 Hz. The performance was very consistent and the voltage stayed the same over the most of the sweep. The TINA curve shows approx 19dB of gain over the spectrum of the audible range. The sound from my Grado SR125 headphones is crisp with a tight lower end. This amp is very nice considering the low voltage and low component count. It may not be true audiophile quality, but the average cost is only $40 USD or less.

Photograph 7: 10Hz Sine Wave

Photograph 8: 100 Hz Square Wave Response

This amp is perfect for the novice builder and the components are available at mouser, digikey. You can substitute other Mosfets as long as the resistance is similar and the internal capacitance does not effect the response curve. It is best to use TINA-TI to build the schematic and make any changes, this way you can check the ac output before building. This will save you much time.

Lastly, make sure that your headphones are not plugged into the jack while powering up or down, this amp as well as other DIY builds have a large rush of current at power up and down and damage can occur to you headphones if you are not careful.

I am sure you will be pleased with the sound of this little amp and better yet the price.